As energy source prices are increasing due to depletion of fossil fuels and interest in environmental pollution is escalating, demand for environmentally-friendly alternative energy sources is bound to play an increasing role in future life. Thus, research into various power generation techniques such as nuclear energy, solar energy, wind energy, tidal power, and the like, continues to be underway, and power storage devices for more efficient use of the generated energy are also drawing much attention.
In particular, demand for lithium secondary batteries is rapidly increasing as mobile device technology continues to develop and demand therefore continues to increase. Recently, use of lithium secondary batteries as a power source of electric vehicles (EVs) and hybrid electric vehicles (HEVs) has been realized and the market for lithium secondary batteries continues to expand to applications such as auxiliary power suppliers through smart-grid technology.
Such lithium secondary batteries generally use metal oxides, such as LiCoO2 and the like, as a cathode active material, and carbonaceous materials as an anode active material. Such lithium secondary battery is manufactured by disposing a polyolefin-based porous separator between an anode and a cathode and impregnating the resultant structure with a non-aqueous electrolyte containing a lithium salt such as LiPF6 or the like. When the lithium secondary battery is charged, lithium ions of the cathode active material are deintercalated therefrom and then are intercalated into a carbon layer of the anode. When the lithium secondary battery is discharged, the lithium ions of the carbon layer are deintercalated and then are intercalated back into the cathode active material. In this regard, the non-aqueous electrolyte acts as a medium through which lithium ions migrate between the anode and the cathode. Such lithium secondary battery basically requires stability within an operating voltage range of a battery, and the capability to transfer ions at a sufficiently high rate.
However, lithium secondary batteries have high operating potentials while having high energy density and discharge voltage and thus high energy may instantaneously flow therein. Accordingly, when a lithium secondary battery is overcharged to 4.2 V or higher, the electrolyte starts to decompose, and when the temperature of the electrolyte increases, the electrolyte may readily reach an ignition point, which results in high possibility of combustion.
In addition, recently, instead of using conventional electrode active materials, research into use of spinel-structure lithium nickel-based metal oxides in cathodes or use of lithium titanium oxides as anode active materials has been conducted.
In particular, spinel-structure lithium metal oxides having formula LiNixMn2−xO4 where x=0.01 to 0.6, which are active materials for high-voltage applications since they have an average voltage of 4.7 V, reach an oxidation potential of an electrolyte and thus the electrolyte is oxidized, resulting in generation of by-products such as gas and the like, which deteriorates secondary battery safety.
Therefore, there is a need to develop a technology that can address these problems.